The present invention relates to a direct-current motor.
In a typical direct-current motor, which has brushes and commutators, the brushes and a commutator change the direction of current supplied to coils, or commutate the current. However, in many cases, in the last stage of switching, the direction of the current is abruptly changed, that is, under commutation occurs. Undercommutation causes the brushes to discharge sparks, which produce noise and wears the brushes.
To prevent undercommutation, commutation is improved by displacing the position of each brush in a direction opposite to the rotation direction of the rotor relative to the circumferential center of the corresponding magnet.
However, the proper position of each brush varies according to the rotation speed of the motor and the current through the coil. Therefore, when the rotation speed and the coil current change due to a change of load acting on the motor, it is difficult to commutate current appropriately. A radical countermeasure has therefore been wanted.
Part of each magnet in a typical direct-current motor is designed to function as a flux changer. The position of the corresponding brush relative to the flux changer influences commutation. If the relative position is inappropriate, occurrence of sparks will be frequent, which prevents improved commutation. Further, the flux changer changes the attraction force generated between the magnet and the armature. Therefore, the rotational force generated when rotating the armature without supplying current to the motor, or the racing torque (cogging torque), is increased. This produces noise and vibration.
Accordingly, it is a first objective of the present invention to provide a direct-current motor that prevents sparks from occurring by locating each brush at an appropriate position relative to the corresponding magnet that has a magnetic flux changer (weak flux part).
A second objective of the present invention is to provide a direct-current motor that reduces cogging torque, thereby operating smoothly with low noise.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a direct-current motor is provided. The motor includes an armature core, a plurality of armature coils, a plurality of magnets, a commutator, and a pair of brushes. The core has a plurality of teeth. The teeth are arranged at a pitch of a first predetermined angle. Each coil is wound about a different group of teeth having a predetermined number of teeth. Each tooth is located at the most advancing position in the rotation direction in one of the teeth groups. The armature core and the armature coils form an armature. The magnets face one another with the armature in between. Each magnet includes a main portion, an extended portion, a first weak flux part. The extended portion extends from the main portion. The first weak flux part is located in the vicinity of the border of the extended portion and the main portion. The first weak flux part extends along one pitch of the teeth, and the flux of the first weak flux part gradually increases along the rotation direction of the armature. The commutator has a plurality of segments. The segments are connected to each coil. The brushes can contact each segment. The brushes supply current to the coils through the segments. During commutation, each brush establishes a short circuit in an adjacent pair of the commutator segments, thereby changing the direction of current flowing through the coil. When commutation is started for a group of teeth, the advancing end of the first tooth in that teeth group, the first tooth being located at the most advanced position in the group in the rotation direction of the armature, is aligned with the first weak flux part of one of the magnets.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.